Processing graphite



Nov. 10, 1936.

K. c. DE WALT PROCESSING GRAPHITE Filed March 17, 1934 eWedt, (A 4 v e n0 0w 56 h t t e S 2% K 9 b Patented Nov. 10, 1936 UNITED STATES mocsssmcGRAPHITE Kenneth C. De Walt, Schenectady, N. Y., assignor' to GeneralElectric Company, a corporation of New York Application March 17, 1934,Serial No. 716,103 7 Claims. (Cl. 250-275) The-present invention relatesto electric discharge devices and more particularly to the materials ofwhich the electrodes of said devices are made.

As the result of considerable research and developmental work, it hasbeen found that graphite or specially treated carbon offers manyadvantages as electrode material over metals which have been mostcommonly employed for this purpose. In the first place, graphite lendsitself readily to well-known extrusion methods so that electrodes ofsimple configuration may be cheaply made. The material is fairly soft sothat if desiredand necessary, the electrode may be easily machined outof solid blocks of graphite. There is less tendency for the finishedgraphite electrode which often is constituted of relatively thinmaterial, to warp than is the case with metal electrodes when subjectedto heat and. consequently, less supporting structure is necessary withinthe tube. Graphite has a relatively high electrical conductivity so thatthere is no appreciable energy loss in the electrode and moreover, has arelatively low vapor pressure so that it produces no deposits oninsulators in the tube which might give rise to excessive electrodeleakage or at leastto blackening of the bulb during exhaust. Finally,graphite offers the most desirable advantage over metal of being able todissipate greater quantities of heat, since it constitutes more nearlyan ideal black body and in view of its much higher melting point thanthat of metal, may be operated at higher temperatures, all of whichresults in providing a tube which gives a higher operating platedissipation.

In contrast to the many advantages which graphite has over metal forelectrode material, it is subject to a most pronounced disadvantagewhich has tended to restrict; its use generally in 'electron dischargedevices. It has been found that there is usually a film or layer of veryfine particles collected on the surface of a graphite electrode, in theform of a powder, and these particles are retained for the most part inthe porous surface structure. When an anode or a control member, orboth, of a high voltage electric discharge device are made of graphite,the electric and thermal forces within the tube tend to detach thesefine particles from the surface and the resulting dust migrates withinthe envelope and often comes to rest on the cathode surface which may bean indirectly heated or a directly heated member coated withthermionically active ,agent. The result of this deposit is to reducethe electron in-water solution emission to a relatively low value andhence to decrease the operating efficiency of the tube.

, In case only the anode is made of graphite and the grid fabricated ofmetal, the graphite particles attracted from the anode may lodge on thegrid or any other electrode in the tube and cause arcing and subsequentbreakdown resulting from the cold emission produced by the intenseelectric fields at the points of the respective particles.

In order to eliminate this conditon, it has been up proposed to brushthe electrode with a steel bristle brush, and to wipe the member with asoft cloth prior to assembly. While this has done much to relieve someof the dimculties attending the use of graphite, there'still remains aconsiderable amount of trouble from the same cause even though specialtreatments on exhaust and added aging schedules of manufacture have beenemployed. It seems that as the voltage between the cathode and anode isincreased, the deleterim ous effects of the graphite powder aremultiplied manyfold so that any process or technique of ridding thegraphite surface still further of these dust particles is highlydesirable, particularly in the field of high voltage devices.

An object of the present invention is to improve the technique oftreating graphite so that the electrode material will be even lesssubject to the production of graphite dust than heretofore; and toprovide such a process as may be readily performed without necessitatingexpensive and elaborate apparatus.

In carrying out this object,

the improved process consists essentially of heating the finishedgraphite electrode, 1. e. one that is ready ggg; to be mounted withinthe tube, in air by induction or in any other suitable manner to about1000 C. A high frequency induction furnace is preferred for this purposesince its heating effect may be readily controlled and reproduced. anAfter the heat treatment for about one minute,

or until substantially all residue has been burned from the electrode,as evidenced by the disappearance of smoke or vapors, the graphite maybe removed from the induction coil or other type 4g of furnace and whilered-hot quenched in a hot non-inflammable liquid such as carbon tetrachloride, acetic acid (28% solution), or a glycer- (25% glycerin), oreven tap or distilled water. It is preferred to have the watermaintained at a somewhat elevated temperature, approximately C. ratherthan at room temperature because the higher the temperature of theliquid, the less sudden will be the quenching efiect of the graphiteelectrode and m the less tendency will there be to break or otherwisedistort the form of the electrode during the cooling process. Forpractical reasons, it is preferred to employ distilled water for thispurpose so as not to introduce any foreign material such as mineralsalts, etc. into the electrode.

Shortly after immersing the graphite electrode in the water, a violentreaction takes place which in effect, blasts the surface free of allimpurities, including the dust particles referred to hereinbefore.The-particles tend to settle in the water and are permanently removedfrom the electrode. As stated hereinbefore, the lower the tempera tureof the water or the higher the temperature of the electrode at the timethe quenching takes place, the more violent will be the blasting action.Consequently,.these temperatures must be predetermined to give theoptimum results, i. e. by producing as violent a reaction as possiblewithout breaking or otherwise distorting the electrode.

The manner in which the electrode surface is freed of impurities is notknown completely at present but in general, I believe that the reactionis produced by a rapid cooling of the surface which permits the water topenetrate the surface. The interior of the graphite is still hot enoughto vaporize the water and produce steam at a pressure sufiicient tocreate the blasting effect. The action literally detaches in a violentmanner the loose dust and impurities from the graphite.

After the reaction is completed, the graphite may be removed from thequenching bath and rinsed in a liquid bath maintained at a somewhatlower temperature, for example, 60 to 70 C. For this purpose, I preferto use distilled water, since it is non-inflammable and does not leavedeposits which are undesirable and is easy to remove in the mannerdescribed hereinafter.

The electrode may then be removed from the rinsing bath and allowed todry in air for a short time, i. e. for approximately hour, to evaporatethe surface water, after which the water absorbed within the interior ofthe electrode may be completely removed by firing the anode by inductionor in any other suitable manner in vacuum or by firing for approximately10 minutes at approximately 1200 C. in a hydrogen atmosphere. Theelectrode is then ready to be mounted on its supporting structure andsealed within the tube. It is found that after having been given thetreatment above described, it is no longer necessary to rub or brush thegraphite. In case the electrode is not to be used for a time, it ispreferable to'wrap the same in cellophane or other protective coveringuntil ready for use.

The apparatus employed in the dust-removing treatment describedhereinbefore is quite simple and inexpensive. Reference should now bemade to the drawing in which Fig. 1 shows in perspective and partly insection, an induction coil and a graphite electrode mounted in positionto receive a heat treatment; Fig. 2 shows the position of the electrodein the quenching bath, while Fig. 3 shows the next step in the processin which the electrode is rinsed in warm water.

In the drawing, numeral l represents a coil having a configuration whichapproximates that of the electrode 2. Oscillatory current obtained froma source not shown flows through the coil and sets up electro-magneticlines which tend to heat any conducting material of a hollow andperimetrically closed configuration contained within the coil, as iswell known in the art. The

electrode 2 is illustrated as taking the form of a well-known type ofanode, which has a general box-like configuration and a thickness in thethinner parts of approximately 1 Intermediate the ends of the box thereare two oppositely directed flanges 3 constituted of solid material.both of which contain a longitudinal opening 4 which is used formounting purposes. In order to apply the high frequency coil or furnaceto the graphite electrode or vice versa, it is preferable to support theelectrode on a metal cradle so that the hands of the operator never haveto come in contact with the electrode either before or after the heattreatment and thereby contaminate the surface thereof. While anysuitable form of cradle will sufiice, I prefer to employ a frame whichincludes a rectangular bar member 5 of metal on which at each end thereis secured a heavy metal washer 6. Projecting upwardly from thesewashers, there is a pair of pins 1 tapered at their uppermost ends,which are so positioned along the bar member as to fit snugly within theopenings 4 already provided in the electrode. It is apparent that theserigidly positioned pins provide lateral support to the thin walls of thehollow electrode. The metal bar 5 carries at the center a rod uprightmember 8 of somewhat longer length than the electrode and whichterminates at the top in an eyelet provided with a slot 9. There is arod member ID, which terminates at one end, in a handle adapted to begrasped by an operator and at the other end, in a flattened hook H whichengages the slot 9.

Obviously, by grasping the handle of the rod I 0, an operator may liftthe entire electrode 2 which rests on the metal bar 5. After having beengiven the high frequency heat treatment described hereinbefore andillustrated in Fig. 1, the elec trode may be removed from the coil andthen plunged in the heated,liquid l2 contained within a receptacle i3 ofsuitable shape and size, main tained in a heated condition by means ofgas jets I4. While in the liquid l2, the graphite electrode undergoes aviolent reaction which removes the dust particles and foreign materialfrom the surface so that the electrode comes from the quenching bath inan extremely clean-condition. The blasting effect is so severe and thestresses set up in the electrode so great that the thin walls would tendto fracture in the absence of the lateral support offered by the cradle.

The electrode may then be lifted from the quenching bath after thereaction has been completed and immersed in the rinsing bath whichpreferably contains distilled water l5, maintained at a lowertemperature than the water I2 in the quenching bath, after which theelectrode is allowed to dry in air for a short time in order toevaporate the surface water and finally, is fired either in a vacuum orin a hydrogen atmosphere as described hereinbefore.

The electrode is ready to be mounted in a vacuum tube and after thetreatment described hereinbefore, is substantially devoid of graphitedust particles or other source of contamination which might give rise todischarge or other irregularities of operation in the tube.

While I have illustrated and to some extent described my invention inconnection with a graphite anode of an electron discharge device, itwill be understood that the invention is not limited to the illustratedform or type of electrode but has application to the treatment ingeneral of graphite or other specially prepared in which case, aspecially designed cradle or supporting frame may be provided, as willreadily 7 occur to those skilled in the art.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is: 1. The method of treating graphite which consists in heatingthe graphite to a temperature at which substantially all residue isremoved by volatilization, then immersing the heated graphite in anon-inflammable liquid so as to cause a violent reaction within thegraphite and to blast the surface free of loose particles.

2. The method of treating a graphite electrode which consists in heatingthe electrode to a temperature at which substantially all residue isremoved by volatilization, then plunging the heated electrode into waterto cause a violent reaction within the electrode and to blast thesurface free of loose particles.

3. The method of treating a graphite electrode which consists in heatingthe electrode to approximately 1000 C., then quenching the heatedelectrode in water to cause a violent reaction within the electrode andto blast the surface free of loose particles.

4. The method of treating a graphite electrode I which consists inheating the electrode by induction to approximately '1000 C., thenquenching the heated electrode in water at a temperature ofapproximately 90 C. to cause a violent reaction within the electrode andto blast the surface free of loose particles.

5. The method of treating a graphite electrode which consists in heatingthe electrode by induction to approximately 1000" C., then quenching theheated electrode in water at a temperature of approximately 90 C. tocause a violent reactionwithin the electrode and to blast the surfacefree of loose particles, then rinsing the cooled electrode in water at atemperature of approximately to C.

6. The method of treating a graphite electrode which consists in heatingthe electrode by induction to approximately 1000 C., then quenching theheated electrode of approximately C., then rinsing the cooled electrodein distilled water at a temperature of approximately 60 to 70 C., dryingthe electrode, and finally firing the electrode in a hydrogen atmospherewhereby the absorbed wateris completely removed.

7. The method of treating graphite which consists in heatingthe graphitesolely in a gaseous atmosphere, then immersing the heated graphite in anon-inflammable liquid to'cause a violent reaction within the graphiteand to blast the surface free of loose particles.

l KENNETH C. DE WALT.

in water at a temperature-

